1. Field of the Invention
The present invention relates to a piezoelectric element, and a liquid ejection head and a recording apparatus which use the piezoelectric element.
2. Description of the Related Art
A piezoelectric material has been employed in a wide field of a piezoelectric element also called a piezoelectric actuator, a liquid ejection head also called a piezoelectric liquid ejection head, and a recording apparatus including the liquid ejection head.
A cross section of a piezoelectric element also called a unimorph piezoelectric element (hereinafter, simply referred to as “piezoelectric element”) is configured, for example, in FIG. 12. In the piezoelectric element, a piezoelectric layer 14 is disposed between a first electrode 13 also called a lower electrode and a second electrode 15 also called an upper electrode, and joined to a vibrating plate 22. In the piezoelectric element, the vibrating plate 22 is deflected and displaced by driving called unimorph driving. FIG. 13 is a schematic diagram for explaining a deflection displacement of the vibrating plate 22 by the unimorph driving. As illustrated in FIG. 13, the magnitude of deflection displacement is determined according to a sum of contributions to two kinds of displacements developed when an external electric field is applied in a direction perpendicular to the vibrating plate 22. The contributions to two kinds of displacements include one contribution to the displacement resulting from crystal strain occurring in the same direction as that of the external electric field, and the other contribution to the displacement occurring in the same direction as that of the external electric field resulting from crystal strain occurring in a direction perpendicular to the external electric field.
In the contributions to the two kinds of displacements, the former is closely related to a piezoelectric constant of d33, and the latter is closely related to a piezoelectric constant d31.
Furthermore, d15 is exemplified as the piezoelectric constant in addition to the above d33 and d31. For example, for PbZr0.55Ti0.45O3 as one of lead zirconate titanate (PZT: PbZr1-xTixO3), values for d33, d31, and d15 are d33=147, d31=−57.3, and d15=399 (×10−12 C/N). In addition, for barium titanate (BaTiO3), the values are d33=191, d31=−79, and d15=293 (×10−12 C/N).
FIG. 14 is a diagram illustrating a relationship of the piezoelectric constants d33, d31, and d15, the external electric field, the crystal strain, and the crystal displacement. As illustrated in FIG. 14, d15 is the piezoelectric constant that is related to a so-called shear strain, and closely related to a displacement developed when crystal strain occurs in the direction of electric field on one plane of the piezoelectric layer which is parallel to the external electric field.
In the piezoelectric element, there is a case in which the vibrating plate cannot be sufficiently displaced by only the deflection displacement as in the related art. This is because an absolute value of the piezoelectric constant is a value as extremely small as the order of about 10−12 C/N. In this case, for example, in the piezoelectric liquid ejection head including the unimorph piezoelectric element, there is a fear that liquid cannot be ejected from a liquid chamber.
In view of the above-mentioned circumstances, there is easily expected that it is important to obtain the large deflection displacement of the vibrating plate through some method.
Among methods of increasing the deflection displacement of the vibrating plate, there is a method involving devising the cross section structure of the piezoelectric element. For example, Japanese Patent Application Laid-Open No. 2000-246888 discloses a liquid ejection head having a convex cross section structure in a width direction of the piezoelectric layer so that neutral planes at both ends may be moved down in the vibrating plate direction to allow an increase in the displacement of the vibrating plate. Furthermore, W. Zhu, N. Li, J. Fu and L. E. Cross, “Proceedings of the 13th US-Japan Seminar on Dielectric and Piezoelectric Ceramics, pp. 81 to 84 (2007)” discloses a piezoelectric element having a lattice structure of a substantially trapezoidal shape in cross section of the piezoelectric layer so as to increase the displacement of the vibrating plate.
However, both of the methods disclosed in the above-mentioned two documents mainly allow d31 to efficiently contribute to the displacement of the vibrating plate, and are suitable for a structure of the piezoelectric element in which only the piezoelectric constants of d31 and d33 are utilized.
As described above, in the piezoelectric material including PZT and BaTiO3, the absolute value of d15 is frequently larger than the absolute values of d31 and d33. Accordingly, it is desirable to propose a piezoelectric element that effectively utilizes d15.